Ultrasonic Non-destructive Evaluation of Titanium Diffusion Bonds

Diffusion bonds offer several advantages over alternative welding methods, including the ability to produce near-net shapes and achieve almost parent metal strength. However, voids remnant from the joining process can be tens of microns in their lateral dimension, making them difficult to detect with conventional pulse-echo immersion inspection at any significant metal depth. In titanium the inspection is particularly challenging; the anisotropic microstructure is highly scattering and the diffusion bond itself forms an interface between regions of preferred crystallographic orientation (macrozones), which can act as a weak spatially coherent reflector. A simple interfacial spring model predicts that, for partial bonds (sub-wavelength voids distributed on the bond line) and at certain frequencies, the phase of the signal can be used to separate the component of the signal due to the change in texture at the interface and the component due to the flaw. Here it is shown that the phase of the signal from an interface is also affected by the anisotropic microtexture of Ti–6Al–4V. Good separation between well-bonded and partially bonded samples was achieved using a symmetric inspection, where the magnitude and phase of the reflection coefficient were calculated for normal incidence from opposite sides of the diffusion bond.